A conventional amperometric determination of chemical components (hereinafter referred to as "analytes") in a solution is performed by immersing a number of electrodes, typically three, in the analyte solution. During electrochemical measurements an electrolyte, such as an ion forming salt or any other ion forming compound which when dissolved increases the conductivity of the solution, is usually added to the analyte solution. The three electrodes are referred to as working electrode (WE), reference electrode (RE) and counter electrode (CE). At least one working electrode is required, but there is no restriction to the use of any number of additional working electrodes, if this is advantageous in a specific desired type of measurement.
At least one working electrode is required, but there is no restriction to the use of any number of additional working electrodes, if this is advantageous for a specific desired type of measurement.
The reference electrode is designed to maintain a constant and well-defined electrical potential. With respect to this potential a fixed electrical potential, negative or positive, is applied on the working electrode by means of an external electronic device, a potentiostat. If analytes in the electrolyte solution being exposed to this potential undergoes reaction at the working electrode, an electrical current will flow between the working electrode and the counter electrode. This current may be measured, and reflects the concentration of the analyte(s) in the solution, while the direction of the measured current flow depends on whether an oxidation or a reduction process is present at the working electrode.
Ideally, no current flows through the reference electrode of a triple electrode system as described above. In the simplest case, the current measured between the working electrode and the counter electrode is directly proportional to the analyte concentration. The chemical reactions occurring at the counter electrode are usually of no interest from a measuring point of view. Therefore, the counter electrode is often separated from the analyte solution and is placed in a separate receptacle in a liquid being electrically connected with the analyte solution via a porous sintered glass filter.
Under certain conditions the reference electrode and the counter electrode may be combined. This is possible when the processes occurring at the counter electrode are reversible with regard to a component in the electrolyte, so that the rate of the electrochemical processes at the combined electrode is not current limiting for measuring an analyte at the working electrode.
A difficulty experienced during amperometric measurements is to achieve well-defined conditions in the solution at the boundary layer adjacent to the working electrode. Such well-defined conditions are of importance for obtaining reliable measurement results from which valid conclusions, such as determination of concentration, may be drawn. The rate at which processes occur at the working electrodes is regulated by the rate of electron transfer across the boundary layer between the analyte in the solution and the electrode, and/or by the rate of analyte transport through the solution to this boundary layer.
In order to achieve a well-defined transport of analyte towards the electrode the working electrode is rotated. Therefore, this is usually formed as a cylindrical body provided with one or several electrodes, usually working electrodes. In a typical embodiment, a central disk-shaped disk electrode and outside this an annular ring electrode, are positioned on the flat end surface whereby both these electrodes are working electrodes. Such a configuration is called a rotating ring-disk electrode.
In addition to working electrodes, reference and counter electrodes could be integrated in the rotating body as well. The present invention is concerned with a measuring device wherein the reference as well as the counter electrode is built into the rotating body, and in its simplest embodiment these are combined to one electrode, herein called RE/CE, under the provision of the situation described above with reversibly working RE/CE. In the borderline case of very low currents through RE/CE such a situation can always be created independently of whether the processes at RE/CE are ideally reversible, or not. In that case, the RE/CE potential is not altered which is the criterion for a good reference electrode. At the same time, currents from the working electrode are taken care of, which is the function of a counter electrode.
Therefore, the measuring conditions that are present in a conventional potentiostat controlled triple electrode system having separate electrodes WE, RE and CE could be achieved with good approximation, so that a double electrode system will be useful in practice for analyzing the content of components in an electrolyte.
A common type of conventional measuring device, for measurements of the type described above, is provided with a rotating electrode fixture on an elongated, hollow shaft. Connection leads extends through the shaft from the electrode surfaces. In their other ends, the connection leads are connected to a collector transferring device, i.e. electrically conducting areas are provided directly on the rotating shaft or as circumferential radial stripes or rings, whereby one such ring is being provided for each electrode, and each ring is being connected to the electrode via a lead, and for each ring a collecting shoe is fixedly mounted with respect to the environment and is connected to a potentiostat.
The U.S. Pat. No. 4,889,608 to G. Eickmann describes an electrode system of the type described above and for the case of one electrode, wherein the electrode is placed in a replaceable electrode fixture, which electrode fixture is mounted on an elongated solid metal shaft via an adaptor, the shaft also acting as an electric conductor. Also with this device electrical voltage and current are transferred to and from the electrode surface via a collector device to an electronic unit for registration of the measured signals.
However, conventional measuring devices of the type related above exhibit disadvantages that reduce their measuring performance.
The currents to be measured are often very small, especially when the analyte concentration is low or when inactive electrochemical processes are studied. However, collector couplings are characteristic in that they tend to generate interferences that are superimposed onto the measuring signal, and at the low analyte currents to be measured these interferences have a perceivable negative influence on the measuring precision.
The subsequent amplification of the measuring signal is of limited value since also the interference is amplified to a corresponding extent.